Energy and nutrient requirements increase during pregnancy because of the following factors:

* Growth of maternal, foetal and placental tissue
* Fat reserves created in the mother which will be used later during lactation

A woman will find that on an average her energy needs will go up to around 150kcal a day in the first trimester and in up to 350 kcal/day in the second or third trimester. Reference: Dietary Fats and Health, by E G Perkins, W J Visek, Pg 279